1. Field of the Invention
The present invention relates to a method and a system for controlling and stabilizing the frequency of a signal generated by an oscillator.
More in particular, the present invention relates to a method and a system for controlling and stabilizing the frequency of a signal generated by a controllable oscillator as temperature changes, so that the oscillation frequency remains extremely stable.
2. Present State of the Art
It is known that many electronic applications require the use of an oscillator, which must be extremely stable in frequency.
For example, one application that requires the use of oscillators being extremely stable in frequency is the broadcasting of the DVB-T digital terrestrial television signal in “SFN” mode, i.e. in a Single Frequency Network, in order to synchronize in frequency transmitters belonging to the same network.
This requirement is typically fulfilled in three different ways:
1) by using metrologic-grade oscillators, e.g. cesium, rubidium and similar units;
2) by using oscillators controllable through an external reference, even if the external reference is at a different frequency than that which must be generated by the oscillator: for example, this category includes oscillators using, as a reference signal, the “1PPS” (“One Pulse Per Second”) signal of GPS (“Global Positioning System”);3) commercial controlled oscillators, e.g. of the “VCXO” (“Voltage Control Crystal Oscillator”) or “OCXO” (“Oven-Controlled Crystal Oscillator”) types.
Each one of the above-mentioned oscillator typologies has its own drawbacks:
1) metrologic-grade oscillators offer excellent accuracy, but they are very expensive and are therefore not suitable for commercial applications;
2) oscillators controllable through an external reference suffer from at least three drawbacks. First of all, the external reference signal requires the installation of external devices, in particular GPS receivers, of dishomogeneous technology with respect to the technology used in the oscillators, so that reliability problems may arise; secondly, the proper operation of the oscillator depends on the availability of the external reference: if the latter is lost, e.g. due to failures or adverse weather conditions, the oscillator may become completely useless; in the third place, if the source of the absolute reference is a satellite system managed by third parties, such as the GPS system, the proper operation of the oscillator depends on parameters out of the user's control. It is in fact known that the use of the GPS system, or of similar satellite systems, is free: however, it is not possible to enter into service contracts to ensure continuity of service or adequate performance levels. In other words, the GPS system is offered “as is”, and may suddenly become unavailable or inadequate or technically insufficient at any time, without the user being allowed to do anything or to raise any objection;3) commercial controlled oscillators have insufficient accuracy for most applications because, in the absence of an external frequency reference, they typically have a daily frequency drift of a few Hz, i.e. a few hundreds Hz or more per year.
For example, in the above-mentioned case relating to the synchronization of transmitters of a DVB-T network operating in SFN mode, the required accuracy is 10−9, i.e. the tolerable frequency drift is a few Hz per GHz.
One of the causes that contributes to the frequency drift of the signal generated by an oscillator is the operating temperature thereof.
It is known in the art, e.g. from U.S. Pat. No. 3,617,955 and U.S. Pat. No. 4,293,830, to use delay lines in order to compensate for drifts caused by temperature. However, said patents do not teach to correct with extremely high precision any frequency drifts due to temperature.